Process for the production of purified terephthalic acid

ABSTRACT

Purified terephthalic acid is prepared by subjecting an aqueous solution of crude terephthalic acid to hydrogenation to reduce impurities, crystallizing this solution to produce a slurry of purified terephthalic acid in an aqueous liquor, and carrying out an integrated separation and washing process. The integrated separation is performed by exchanging the acidic reaction medium with water to produce a water containing cake of terephthalic acid.

This application was filed under U.S.C. 371 from the application filedMay 19, 1993.

This invention relates to the production of terephthalic acid.

Purified terephthalic acid (PTA) is commonly produced by purification ofcrude terephthalic acid (CTA) obtained by the liquid phase air oxidationof p-xylene. It may also be produced by the hydrolysis of a polyalkyleneterephthalate, for example polyethylene terephthalate.

Such purification typically comprises adding an aqueous medium to theCTA to form a slurry thereof which is then heated to dissolve the CTA inthe medium to provide an aqueous solution of terephthalic acid. Thissolution is then passed to a reduction step in which the solution iscontacted with hydrogen under reducing conditions in the presence of aheterogeneous catalyst to reduce chemically organic impurities, forexample 4-carboxybenzaldehyde (4-CBA). The hydrogenated solution ispassed to pressure let-down vessels in which PTA crystals form toprovide a slurry of PTA in aqueous medium. PTA is recovered from theaqueous medium.

It is known to recover PTA from the aqueous medium by separating the PTAand medium in a decanter centrifuge in a first separation stage,reslurrying the PTA with fresh water, separating the reslurried PTA andfresh water in a decanter centrifuge in a second separation stage toproduce a PTA cake and then drying the cake in a drier.

These steps involve special equipment which incur significant capitalcosts and variable costs of operation. The recovery stage requires largequantities of water in the centrifuge and reslurry steps. Therefore alarge quantity of aqueous waste effluent must be treated and may presentproblems of disposal. Further, dissolved PTA and other valuablematerials in the waste effluent stage may be lost.

The present invention seeks to provide improvements in the production ofterephthalic acid.

According to one aspect of the present invention there is provided aprocess for the purification of crude terephthalic acid, comprising:

(a) dissolving the crude terephthalic acid in an aqueous medium toproduce a terephthalic acid-containing solution;

(b) contacting the terephthalic acid solution with hydrogen underreducing conditions and at elevated temperature and pressure to reducechemically at least some of the impurity present in the crudeterephthalic acid;

(c) reducing pressure and temperature to obtain a slurry comprisingcrystallised pure terephthalic acid in aqueous medium;

(d) in a first zone effecting separation of the PTA crystals from theaqueous medium by filtration of the slurry to remove aqueous mediumthrough a filter surface to obtain a wet mass of crystals;

(e) transferring said wet mass of crystals to a second zone withoutreslurrying said mass and supplying aqueous wash liquor to said masswhile effecting filtration whereby the wash liquor is displaced throughthe mass and through a filter surface in the second zone; and

(f) maintaining a differential pressure across the filter surface in atleast one of said zones such that, on the lower pressure side of thefilter surface, the pressure is substantially the same as or greaterthan the pressure prevailing following step (c).

Step (c) will normally be carried out in a series of crystalliserstages, in which the pressure and temperature is reduced progressively.Thus, the pressure differential of step (f) will therefore be such that,on the lower pressure side of the filter surface, the pressure is atleast equal to the pressure prevailing in the final crystalliser stage,which will normally be superatmospheric, e.g. within the range 1.5 to 15bara, more preferably 3 to 10 bara.

Preferably the pressure differential will be such that, on the lowerside of the filter surface in each of said zones, the pressure is atleast equal to the pressure prevailing following step (c).

Advantageously steps (d), (e) and (f) are carried out by dischargingsaid slurry on to a filter material which is movable to transport theterephthalic acid through said first zone in which filtration of saidaqueous medium from the slurry is effected, to the second zone in whichsaid mass of crystals is washed by displacement of said aqueous washliquor through said mass.

In this way, reslurry of the terephthalic acid is avoided and byeffecting filtration through a filter surface in such a way that thelower pressure side of the filter surface is at a pressure no less thansaid superatmospheric pressure, liquid removal from the terephthalicacid can be effected substantially without accompanying flashing therebyreducing the tendency for soluble impurities to precipitate andcontaminate the mass of purified terephthalic acid. In addition, anytendency for material to precipitate and foul the filter medium isreduced.

The filter material is suitably a metal gauze, or a cloth comprising aplastics material such as polyester, polypropylene, polyetheretherketone(PEEK) and the like. The filter surface is suitably in the form of aband, preferably a continuous band which is moved continuously orintermittently to convey material comprising terephthalic acid throughthe first and second zones.

The second zone may be followed by a third zone in which the washeddeposit of PTA is removed from the filter surface by any suitable means,for example by scraping or by gravity.

Further, it is desirable to provide suitable means to pass liquid forexample water or alkaline solution, through the returning part of theband to wash off downwardly facing adhering deposits into a receiver.

Usually the CTA employed in the production process of the invention isderived from the oxidation of paraxylene in a liquid reaction mediumcontaining acetic acid to produce a slurry of CTA in the reactionmedium. The liquid reaction medium normally incorporates a catalyst, forexample a cobalt/manganese/bromide catalyst system which is soluble inthe reaction medium. Suitably the oxidation is carried out in thepresence of an oxygen source for example air, at a pressure of 5 to 30bars absolute, and preferably an oxygen concentration of 0 to 8% byvolume in the gas leaving the reactor and at a temperature of 150° to250° C. It is suitably a continuous process, and is preferably carriedout in a stirred reactor. The reaction is exothermic and the heat of thereaction may conveniently be removed by evaporation of water and aceticacid from the reaction medium.

The water and acetic acid evaporated from the reaction medium ispreferably distilled to produce acetic acid having a lower watercontent. Acetic acid having a lower water content obtained in this waymay be passed to the oxidation step and the water recovered fromdistillation may be used as the aqueous medium for dissolving CTA and/oras the aqueous wash liquor for washing PTA.

Following the oxidation step, the CTA produced may then be separatedfrom the reaction medium conventionally by centrifugal separation anddrying but it is more expedient to exchange the reaction medium,preferably continuously, for an aqueous medium to provide a terephthalicacid stream comprising CTA in aqueous medium in a process such as thatdisclosed in our prior co-pending EP-A-502628.

If desired, following combination of the CTA with aqueous medium toproduce the terephthalic acid-containing solution, the resultingsolution may be fed directly to the hydrogenation step or alternatively,may be treated prior to it being fed to the hydrogenation step. Suchtreatment may comprise subjecting the aqueous terephthalicacid-containing solution to oxidation to increase the conversion ofterephthalic acid precursor compounds, especially 4-CBA, intoterephthalic acid. Such oxidation my be effected in the aqueous phaseusing air or gaseous oxygen or a non-gaseous oxidising agent may beemployed.

Suitably the heterogeneous catalyst employed in the purification of thecrude terephthalic acid product is a supported noble metal catalyst, forexample platinum, rhodium and/or preferably palladium on an inert, forexample carbon, support. The reduction is suitably carried out bypassing the terephthalic acid solution comprising terephthalic acid andimpurities, for example 4-carboxybenzaldehyde, through a flooded bed ofcatalyst at a temperature of 250° to 350° C. in the presence ofhydrogen. The solution suitably comprises 20 to 50% by weight ofterephthalic acid.

The terephthalic acid solution, after reduction, is suitably cooled in acrystallisation process to a temperature in the range 100° to 220° C.,typically 135° to 180° C., and a pressure of 3 to 10 bara, to producesolid purified terephthalic acid product.

Desirably, at least part of the aqueous medium removed in the first zoneand at least part of the aqueous wash removed through the filter surfacein the second zone is recovered and combined, directly or indirectly,with the CTA. The aqueous wash and/or medium thus desirably constituteat least a part of the aqueous medium with which the CTA is combined.

If both the aqueous medium and the aqueous wash are recycled, they maybe mixed together to form a single stream prior to combination with theCTA. The aqueous medium and the aqueous wash may be treated, for exampleby distillation and/or evaporation to produce substantially pure wateror at least partially eliminate para-toluic acid, either individuallybefore mixing, or as a single stream after mixing prior to beingcombined with the CTA as desired.

Such treatment of the aqueous medium and/or wash may also comprisecooling, preferably to a temperature in the range 15° to 100° C., orevaporation of the aqueous medium and/or wash to produce a less pureprecipitate and a residual mother liquor which are then suitablyseparated. Suitably, where the CTA is produced by an oxidation plantintegrated with the purification plant, such less pure precipitate isreturned to oxidation step of the oxidation plant. The mother liquor maybe treated further and/or used as aqueous medium to be combined with theCTA.

The second zone desirably comprises a single stage wash in which thewash liquor passes through the filter surface only once either as asingle stream or, following splitting of the wash liquor, as a pluralityof streams. If desired the second zone may comprise a succession of washstages wherein the wash liquid is passed through the filter surface morethan once. The succession of wash stages may be co-current but ispreferably counter-current in which, in each stage (other than thelast), the incoming aqueous wash passed through the wet mass of crystalsand the filter surface is the aqueous wash which has passed through thewet mass and the filter surface in the succeeding stage. In the laststage the aqueous wash liquor is preferably fresh incoming water.

The wash liquor is preferably introduced at a temperature which issubstantially the same as the temperature of said mass as it enters thesecond zone so as to avoid problems with flashing or quenching (with theconsequent risk of precipitating impurities).

The wash liquor is suitably at least in part water separated from aceticacid in the aforesaid distillation step following evaporation of aceticacid and water from the oxidation step, if present, or derived fromother water streams within the process, for example from treatment ofthe aqueous wash and/or medium. This is advantageous as it reducesfurther the intake of fresh water and disposal of water in the process.

Typically the pressure differential across the filter surface in each ofsaid zones is at least 0.05 bar with the side of the filter surface onwhich the mass of terephthalic acid crystals is located being at ahigher pressure than the other side of the filter. Preferably thepressure differential is 0.1 to 10 bar, more preferably, 0.2 to 3 barand especially 0.2 to 1 bar, for example 0.3 bar.

The actual pressure on the lower pressure side of the filter ismaintained at such a pressure that the aqueous wash liquor in the secondzone and, if applicable, the aqueous medium in the first zone which areremoved through the filter surface, remain substantially in the liquidphase.

The higher pressure side of the filter surface is preferably maintainedat elevated pressure, desirably at 2 to 15 bara and especially 3 to 10bara and is desirably above the pressure of the precedingpressure-reducing step in the process.

The terephthalic acid slurry is suitably introduced into the first zoneat a temperature of at least 60° C. and preferably 100° to 200° C.,especially 120° to 180° C.

Suitably the slurry is deposited in such a way that the saturationpressure of the feed is less than the absolute pressure on the lower(downstream) side of the filter medium.

Deposition of the terephthalic acid stream at elevated temperature andpressure is advantageous as improved filtration is possible due to theaqueous medium being less viscous at elevated temperature. Furthermorethere is less co-crystallisation of impurities for example p-toluicacid, with the terephthalic acid product at elevated temperature. Thus ahigher purity terephthalic acid product is obtained and there is acorrespondingly higher level of impurities for example p-toluic acid inthe aqueous medium which is desirably recycled within the process. Theelevated temperature may also permit heat recovery and hence provide areduction in variable costs.

If recycle of aqueous medium and/or wash liquor is employed, it may benecessary to provide a purge to allow control of the level of componentsin the recycle stream.

According to a second aspect of the invention there is provided aprocess for the production of terephthalic acid comprising:

(a) effecting oxidation of paraxylene in a reaction medium comprising analiphatic carboxylic acid to produce a first slurry of crudeterephthalic acid in the reaction medium;

(b) subjecting said first slurry to a separation and washing process inwhich the reaction medium is removed from the crude terephthalic acid;

(c) dissolving the crude terephthalic acid from step (b) in an aqueousmedium to produce a terephthalic acid-containing solution;

(d) contacting the terephthalic acid solution with hydrogen underreducing conditions and elevated temperature and pressure conditions toreduce chemically at least some of the impurity present in the crudeterephthalic acid;

(e) reducing pressure and temperature to obtain a second slurrycomprising crystallised purified terephthalic acid in aqueous mediumcontaining reduced impurities in solution; and

(f) subjecting the second slurry to a separation and washing process inwhich said aqueous medium is removed from the purified terephthalic acidwith the aid of an aqueous wash liquor and without reslurrying thepurified terephthalic acid prior to washing thereof with the aqueouswash liquor;

the separation and washing process carried out in each of steps (b) and(f) being implemented under elevated pressure conditions using arespective belt filter system.

Usually the second slurry will be at superatmospheric pressure followingstep (e).

The amount of water required in the process according to said secondaspect of the invention is significantly reduced as compared with thatrequired in prior art processes involving a reslurry operation. Animportant factor in this respect is the use of belt filters; suchfilters allow highly efficient washing to be carried out and because theseparation of terephthalic acid from the reaction medium or aqueousmedium can be integrated and carried out on the same filter belt, theneed for reslurry of the terephthalic acid is eliminated. Overalltherefore the amount of water employed in the terephthalic acidproduction process can be substantially reduced. In turn, the amount ofwaste effluent to be treated can also reduced substantially. Largequantities of effluent preclude the recycle of a substantial portionthereof into the terephthalic acid stream without further treatment. Insaid second aspect of the present invention a reduction in the amount ofwater required to provide a given wash efficiency allows liquid, hereinreferred to as mother liquor, recovered from the purification plant, forexample aqueous medium from the separation of purified terephthalic acidproduct and the aqueous wash, to be recycled into the terephthalicproduction process. Such recycling is highly desirable as valuablematerials in the mother liquor may be retained within the process ratherthan lost by disposal.

The improved washing efficiency allows higher impurity levels in theterephthalic acid stream to be tolerated. Consequently, the impuritylevel in the CTA which is fed to the hydrogenation step may be higherthus allowing beneficial re-optimisation of the process by which the CTAwas produced, for example by reducing the severity of the oxidationreaction conditions. Further, the hydrogenation step and, if desired,the pressure and temperature reducing step may be also be re-optimised,if desired, such that the terephthalic acid stream has higher impuritylevels.

According to a further aspect of the present invention, a process forthe production of terephthalic acid comprises:

(a) effecting oxidation of paraxylene in a reaction medium comprising analiphatic carboxylic acid to produce a first slurry of crudeterephthalic acid in the reaction medium;

(b) subjecting said first slurry to an integrated separation and washingprocess in which the reaction medium is removed from the crudeterephthalic acid and the crude terephthalic acid is washed with aqueouswash liquor;

(c) dissolving the crude terephthalic acid from step (b) in an aqueousmedium to produce a terephthalic acid-containing solution;

(d) contacting the terephthalic acid solution with hydrogen underreducing conditions and elevated temperature and pressure conditions toreduce chemically at least some of the impurity present in the crudeterephthalic acid;

(e) reducing pressure and temperature to obtain a second slurrycomprising crystallised purified terephthalic acid in aqueous mediumcontaining reduced impurities in solution;

(f) subjecting the second slurry to an integrated separation and washingprocess in which said aqueous medium is removed from the purifiedterephthalic acid and the purified terephthalic acid is washed withaqueous wash liquor;

(g) treating the aqueous medium removed in step (f) to produce a lesspure precipitate of terephthalic acid and a residual aqueous motherliquor; and

(h) recycling said less pure precipitate to the oxidation step (a)and/or recycling said residual mother liquor for use in step (c).

By "integrated separation and washing process" we mean that theseparation and washing steps are carried out within the same item ofequipment without an intermediate slurrying of the terephthalic acid inwater prior to the washing step.

In one embodiment of said further aspect of the invention, theseparation and washing process carried out in each of steps (b) and (f)is implemented under elevated pressure conditions using a respectivebelt filter system.

In another embodiment of said further aspect of the invention, theseparation and washing process carried out in each of steps (b) and (f)may be implemented in the case of the separation stage in a first zoneinvolving deposition of the terephthalic acid on a solid inner surfaceof a rotatable hollow vessel, for example a centrifuge, and in the caseof the washing stage a filter surface forming a second zone of thevessel. The solid surface and the filter surface are suitably arrangedsuch that the mass of purified terephthalic acid crystals is transportedalong the inner surface across the filter surface to a third zone, forexample by a scroll on the inner wall of the hollow vessel, from whichit is recovered and the aqueous medium from the terephthalic acid streamis transported away from the solid inner surface without crossing thefilter surface as a result of the rotation of the hollow vessel, therebyeffecting separation of the purified terephthalic acid and the aqueousmedium. As the solid purified terephthalic acid passes over the filtersurface, it is washed with an aqueous wash liquor which is removedthrough the filter surface to produce washed mass of terephthalic acidcrystals. The washed mass is suitably transported to the third zone byrotation of the vessel and subsequently recovered from the third zone.

In practice, it is desirable to establish an atmosphere of inert gasover the filter cake in order to exclude or control the level of oxygenpresent. This inert gas also passes through the filter material in thewashing zone and additional gas must be introduced in order to maintainthe gaseous atmosphere. The inert gas may be nitrogen for instance,although in the case of the filter/washing system employed in thepurification stage of the process, the inert gas may with advantagecomprise steam.

Thus for instance the process of the invention as defined in any of theabove aspects thereof preferably comprises means for supplyingpressurised gas to the washing zone to establish a pressurised gaseousatmosphere on that side of filter surface on which the layer or mass orterephthalic acid crystals is formed. The gas from said atmosphere willbe drawn through the mass of terephthalic acid crystals along with thewash liquor and preferably means is provided for recovering the gas andtreating it to eliminate or at least reduce contamination thereof by theliquor, and the treated gas may be reintroduced into said atmosphere.

At least the separation step, and preferably the washing step also, iscarried out in such a way as to prevent any substantial precipitation ofsolute from the liquid component. For instance, the liquor component inthe purification stage may comprise a substantial content of solute inthe form of paratoluic acid which should desirably be maintained insolution.

Thus, according to a preferred feature applicable to all of the aspectsof the invention defined above, the pressure differential establishedbetween the upstream and downstream sides of the filter medium iscreated by means of a pressurised fluid constituted at least in part bya solvent component of the liquor, which component is in its vapourphase whereby flashing of the solvent component actually present in theliquor is substantially suppressed during filtration of the terephthalicacid/liquor slurry.

By employing a pressurised fluid based on the solvent component actuallypresent in said liquor or said liquid , it is possible to conduct thefiltration in such a way that the filter cake is not chilled or at leastonly cooled to a somewhat lesser extent than is the case where thepressurised fluid comprises nitrogen or other inert gas.

It is highly desirable that the partial pressure of said solventcomponent in or forming the pressurised fluid on the downstream side ofthe filter medium is maintained substantially at or above the vapourpressure of the solvent component actually present in the slurry fed tothe upstream side of the filter medium.

The solvent component employed as said pressurised fluid may be derivedfrom a product stream from which the liqour to be filtered is derived.

Thus, for instance where the process of the present invention isemployed for the filtration of the product stream resulting from fromthe liquid phase oxidation of para-xylene in acetic acid as solvent,filtration of the slurry may be effected using acetic acid vapour as thepressurised fluid. The acetic acid may, but need not necessarily, bederived from the acetic acid evaporated from the liquor during acrystallisation process (which may be conventional) carried outfollowing withdrawal of the acetic acid/terephthalic acid slurry fromthe oxidation reactor. In this case therefore, following thecrystallisation process in which further terephthalic acid crystallisesout from the liquor, the slurry is passed to a filtration zone in whichthe method according to the invention is carried out using, as thepressurised fluid, acetic acid vapour derived from the crystallisationprocess.

Similarly where terephthalic acid (however obtained) is purified bydissolving the same in water and subjecting the solution tohydrogenation, the purified terephthalic acid is subsequently recoveredby filtration using water vapour as the pressurised fluid, i.e. in theform of steam. Again the steam may, but need not necessarily, be derivedfrom a crystallisation process following the purification process.

The pressurised fluid may consist substantially wholly of said solventcomponent in the vapour phase but we do not exclude the possibility ofincorporating in the pressurised fluid a non-condensible gas (usuallyone which is inert with respect to the liquor/crystals system, e.g.nitrogen). By non-condensable, we mean a gas which is only condenses attemperatures somewhat below normal room temperature and pressure.

Where a mixture of the solvent component and a gas are employed, thecomposition should desirably be such that the partial pressure of thesolvent component in the pressurised fluid is substantially at or abovethe vapour pressure of the solvent component in the slurry at the feedcondition of the latter.

Usually the solvent component will comprise the major fraction (byvolume) of said pressurised fluid.

Preferably the filtration is carried out in such a way that thetemperature differential across the filter cake is essentially zero, andtypically no more than 1° to 2° C.

Other more specific features of the process defined with reference tosaid first aspect of the invention are also optionally incorporated insaid second and third aspects where the context admits. In some cases,for instance where the terephthalic acid-containing aqueous solution issubjected to an oxidation reaction prior to step (d) of said second orthird aspect of the invention in order to oxidise terephthalic acidprecursors such as 4-CBA to terephthalic acid, the lower pressure sideof the filter may be at or below atmospheric pressure if desired.

The invention will now be described by way of example only withreference to the accompanying drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is flow sheet of a terephthalic acid production process inaccordance with the present invention;

FIG. 2 is a schematic view of a belt filter system for use in theprocess of FIG. 1 and which integrates separation and washing stages;

FIG. 3 is a diagrammatic cross-sectional view of one form of centrifugefor implementing integrating separation and washing stages;

FIG. 4 is a diagrammatic view of an alternative form of centrifuge foruse in a process in accordance with the invention;

FIG. 5 is a schematic diagram showing one form of filtration system inwhich pressurisation on the upstream side of the filter medium iseffected by means of a solvent component forming part of the liquor inwhich terephthalic acid is slurried; and

FIG. 6 is a schematic diagram showing another form of filtration systemin which pressurisation is effected by means of a solvent componentpresent in the liquor.

Referring to FIG. 1, reactor A is fed with paraxylene and acetic acidcontaining a dissolved catalyst comprising cobalt, manganese and bromineions by line 1 and with air via line 2. Product from the reactor A ispassed to crystallisation section B by line 3. The temperature withinthe reactor A is controlled by evaporating a mixture of acetic acid andwater from the reactor to a condensing system C via line 4 and istypically above 150° C. Most of the condensate is returned to thereactor A via line 5 with noncondensibles venting via line 6. To controlthe water content of the reactor A, part of the condensate is removedfrom the condensing system via line 7 and passed to the distillationcolumn D.

In the crystallisation section B the temperature is dropped toapproximately 80° C. to 150° C. and the slurry containing crystallineterephthalic acid in reaction mother liquor (mainly acetic acid) therebyproduced is passed to a separation stage E. Acetic acid and/or water maybe recovered from crystallisation section B and passed to thedistillation column D via line 8 and/or to the reactor A via line 9a.The crystallisation section B typically comprises a series ofcrystalliser vessels in which the pressure and temperature of the slurrywithdrawn from the oxidation reactor A is progressively reduced. In atypical embodiment of the invention, the resulting slurry of crudeterephthalic acid in acetic acid-based mother liquor is reduced to apressure in the range 0.5 to 2 bara and a temperature in the range of90° to 130° C.

Separation stage E may be a conventional centrifuge and drierarrangement but is preferably an integrated separation and washing stagein which a continuous solvent exchange process is carried out, e.g. asdisclosed in our prior EP-A-502628. Reaction mother liquor recoveredfrom stage E is returned in part via lines 9 and 9a to the reactor Aoptionally by first mixing with the fresh catalyst, paraxylene andacetic acid contained in line 1. Any remaining reaction mother liquorand any wash liquid is suitably passed to an evaporation stage F inwhich water and acetic acid vapour is removed by line 10, condensed andpassed to reactor A via line 10a or optionally passed to distillationcolumn D via line 10b. A purge of by-products and catalyst is withdrawnvia stream 11.

The solids material, i.e. crude terephthalic acid (CTA), recovered fromthe separation stage E is transferred to reslurry stage G. In reslurrystage G, the CTA crystals are reslurried with water recovered from thedistillation column D via lines 12, 12a and/or other water which may berecycle mother liquor via stream 13, recycle mother liquor via stream 14and/or demineralised water via stream 15. The slurry produced in thisstage is heated in section H to a temperature of for example 250° C. to350° C. to form an aqueous solution of CTA which is passed to reactor Jin which it is reacted with hydrogen over a fixed bed palladium catalystthus reducing impurities in the solution and then crystallised incrystallisation section K. The temperature to which the solution iscooled in the crystallisation section K and the rapidity of cooling isadjusted to produce the appropriate purity of the desired terephthalicacid product. The crystallisation section K typically comprises a seriesof crystalliser vessels in which the pressure and temperature of thehydrogenated solution is progressively reduced. In a typical embodimentof the invention, the resulting slurry of purified terephthalic acid inaqueous mother liquor is reduced to a pressure in the range 3 to 10 baraand a temperature in the range of 135° to 180° C.

The slurry from the final stage of the crystallisation section K istransferred at the pressure and temperature prevailing in the finalcrystalliser stage to section L in which an integrated separation andwashing process is carried out. Thus, in stage L, PTA crystals areseparated from the aqueous mother liquor and the separated PTA productis washed with water which may be derived from column D via lines 12,12b, recovery stage M via line 17 and/or fresh water via line 18 and isrecovered, following washing, via line 19. The aqueous mother liquorfrom the separation in stage L is passed to recovery stage M via line13a and/or to reslurry stage G via line 13.

In stage M the aqueous mother liquor is evaporated or further cooled soas to permit the recovery of further solids in the form of a less pureprecipitate of terephthalic acid which is passed back to reactor A viastream 20. In stage M the temperature of the liquor may be reduced byflashing steam from it at, for example, atmospheric pressure. Such steammay be further purified for example by distillation in column D via line22 and used if desired as wash in stage L, used elsewhere in the processor purged. The remaining liquor may be cooled or evaporated further andsolids separated from it and recycled to reactor A via line 20 asdesired.

The mother liquor recovered from stage M may be in part passed back tothe distillation column D via line 22 and processed as described later,may be returned to the reslurry stage G via stream 14 and/or be purgedvia stream 21. Preferably, if the aqueous mother liquor is evaporated,the evaporated water is returned to the reslurry stage G via line 14.

The distillation column D fractionally distils a mixture of water andacetic acid evaporated from the reaction medium in condenser system Cand is modified if required for use for the treatment of mother liquorseparated from stages F and M . The column D comprises three zones; anupper zone comprising for example 5 theoretical stages, a middle zonecomprising for example 45 theoretical stages and a lower zone comprisingfor example 5 theoretical stages. Part of the mixture of acetic acid andwater evaporated derived from the reactor A is passed via stream 7optionally together with stream 8 and/or 10b to between the middle andlower zones of the column D. Mother liquor from the precipitation ofterephthalic acid may be passed into the column D between the upper andmiddle zones via stream 22. Acetic acid and heavy material are passedfrom the base of the column D via stream 23 to reactor A. Water iscondensed in the condenser and may be re-used in the process via stream12.

Referring to FIG. 2, there is shown one embodiment for implementing anintegrated separation and washing process suitable for use in each ofstages E and L, in the form of a continuous band or belt filter unitsuch as a Pannevis filter of the form generally described in Filtrationand Separation (Page 176 et seq, March/April 1979). The filter unitcomprises an endless filter belt or band 100 driven by rollers aroundwhich the belt or band extends at each end, the belt being enclosed in apressure tight housing 101. The belt 100 comprises generallyhorizontally disposed upper and lower runs 100a and 100b. The interiorof the housing 101 is pressurised with a suitable gas such as nitrogenin the case of the filtration unit employed in stage E or nitrogen orsteam in the case of the filtration unit employed in stage L.

Dotted lines S and T show the locations of a first zone on the left, asecond zone between the lines S and T and a third zone to the right ofline T. The slurry of terephthalic acid in mother liquor (i.e. aceticacid in the case of stage E or aqueous medium in the case of stage L) isintroduced to the first zone via line 103 onto the band and motherliquor drains through the band into collector pan 104 from which it isremoved via line 105 to leave a first wet deposit of terephthalic acidcrystals which is then passed to a second (middle) zone (in thedirection of arrow A). In the second zone, aqueous wash liquor, forexample water, is introduced via line 106 and passed through the band tocollector pan 107 to produce a second wet deposit. The aqueous wash isremoved via line 108. The second wet deposit then passes to a third zone(in the direction of arrow B) in which it is removed from the band,collected in a receiver 109, recovered and then slurried with aqueousmedium in the case of CTA or dried in the case of PTA.

The water introduced via line 106 may be derived from line 12b, 17and/or 18 (stage L) or line 12c (stage E) as shown in FIG. 1 or anyother suitable source. Suction is applied to the filter cake formed onthe upper run 100a via the pans 104 and 107 and the pans are coupledtogether for reciprocating movement as a unit in a direction parallel tothe direction of travel of the belt. During travel of the pans from leftto right, suction is applied to draw liquid through the upper run 100aand during return travel of the pans, suction is terminated. Thepressure differential across the filter belt, between the region abovethe filter cake and the downstream side of the filter medium (i.e. theinterior of the pans) is typically of the order of 0.6 bar. In the caseof stage L, the pressure residing on the downstream side of the filtermedium is at least as great as that prevailing in the final stage of thecrystallisation section K. The wash liquor applied to the filter cakepasses through the cake by virtue of the pressure differential betweenthe upstream side of the filter cake and the downstream side of thefilter medium and is applied in such a way that the wash liquordisplaces residual mother liquor within the cake without undergoingchannelling. It will be appreciated that the Hash liquor may be appliedin a single stage or it may comprise a series of stages with wash liquorapplied at a number of locations along the path of travel of the filterbelt. In this event, the wash liquor may be applied as a number ofparallel streams or it my be applied in a serial manner, eitherco-current or counter-current. The mother liquor and wash liquorrecovered from each filter belt unit are utilised in the mannerdescribed with reference to FIG. 1. Thus, for example, in the case ofthe filtration unit forming stage L, the mother liquor and recoveredwash liquor may be combined and then subjected to cooling or evaporationto produce a less pure terephthalic acid precipitate which is recycledto the reactor A. The residual mother liquor/wash liquor may be furtherprocessed for example to reduce the para-toluic acid content thereof andthen recycled to the reslurry stage G for use in dissolution of CTAprior to the hydrogenation reaction in reactor J.

The stages E and L may be implemented by other forms of filtration unitcapable of operating under pressure; for instance, the filtration unitsmay be constituted by pressure drum filters, e.g. multi-cell pressuredrum filters of the type well-known in the filtration art or rotarycylindrical filters such as the type disclosed in EP-A-406424.Alternatively the integrated separation and washing units forming stagesE and L may take other forms such as those described below.

FIG. 3 shows a cross-section representation of a centrifuge adapted foruse in the present invention which comprises a hollow vessel 200 ofcircular cross section which is rotatable about its longitudinal axis Rand which comprises an inner solid wall 201 onto which the terephthalicacid stream comprising terephthalic acid in an aqueous medium isdeposited via inlet 202. Dotted lines S and T denote the boundariesbetween the zones, the first zone being above line S, the second zonebeing between lines S and T and the third zone being below line T.Rotation of the vessel causes a substantial proportion of the motherliquor component of the terephthalic acid slurry to be transported inthe direction of arrow C, out of the first zone whilst the purifiedterephthalic acid-containing wet deposit is transported in the directionof arrows D, across the second zone and across the third zone by meansof an inner scroll 203. In the second zone, the wet deposit passesacross filter surface 204 and is washed with an aqueous wash liquorsupplied by inlet 205. The aqueous wash liquor may be derived fromsources such as those described in relation to the embodiment of FIG. 2.The Wash liquor is withdrawn through the filter surface 204 and the wetdeposit is retained by the filter 204 to produce a second wet depositcomprising terephthalic acid which is transported to the third zone inthe direction of arrow E. The second wet deposit is then desirablyremoved from the third zone and processed further, i.e. by slurrying instage G or drying depending on whether the terephthalic acid is in itscrude or purified form.

FIG. 4 shows a cross section representation of another form ofcentrifuge adapted for use in the present invention, the same referencenumerals being used in FIGS. 3 and 4 to depict like components. Thecentrifuge of FIG. 4 differs from that of FIG. 3 in several respects.Inlets 202 for depositing the terphthalic acid stream and inlet 205 forproviding the aqueous wash are coaxially arranged along the longitudinalaxis of the centrifuge and an additional aqueous wash inlet 206 isprovided which is also coaxial with inlets 202 and 205. Inlet 202 has agreater diameter than inlet 205 which, in turn, has a greater diameterthan inlet 206. A further difference between FIGS. 3 and 4 is that thesecond zone (between S and T) is sub-divided into a dewatering zonebetween V and T, and into two wash sub-zones between U and V, and S andU respectively. As the wet deposit of terephthalic acid is transportedacross the filter surface 204 in the sub zone between S and U, it iswashed with aqueous wash via inlet 205 which wash has been obtained asthe filtrate recovered from the succeeding sub-zone between U and V. Asthe deposit passes through the sub zone between U and V, it is washedwith aqueous wash, preferably demineralised water, via inlet 206. Thus,the wet deposit is washed counter-currently as it passes through thepart of the second zone. In the sub zone between V and T, the washed wetdeposit may be dewatered by removing aqueous wash through the filtersurface 204 to produce a second wet deposit which is then passed to thethird zone in the direction of arrow E.

As the separation and washing processes are carried out under pressuresomewhat in excess of atmospheric pressure, where it is desired totransfer the washed terephthalic acid to equipment operating at lowerpressure (e.g. at atmospheric pressure), this may be effected either byreslurrying the filter cake or by transferring the filter cake in itsmoist condition from higher pressure to lower pressure using a suitablepressure isolating arrangement. For instance, a PTA filter cake removedfrom the filtration unit of FIG. 2 into the receiver 109 may betransferred to drying equipment at lower pressure by means of variousdevices such as variable pitch screws, the use of two lock vessels, oneof which can be under pressure and filled with filter cake and the otheropen to atmosphere, or a rotary valve arrangement.

Referring now to FIG. 5, this illustrates the use of a pressurisingfluid for the filtration step in such a way as to reduce further anytendency for solute present in the slurry liquor to precipitate out andhence contaminate the terephthalic acid. As shown, slurry stored in atank 310 is fed by pump 312 and flow controlled valve 314 to a rotarydrum type filter 316 which may be of conventional design. The slurry isfed to the tank 310 via line 318 and may be derived for example from acrystalliser system (not shown) associated with plant for effectingpurification of terephthalic acid by contacting an aqueous solutionthereof with hydrogen in the manner previously described. The rotaryvacuum filter 316 comprises an outer housing 320 within which acylindrical support 322 carrying a filter medium (typically a filtercloth) is mounted for rotation about a horizontal axis perpendicular tothe plane of the paper. The slurry is supplied to the lower region ofthe housing 320 which forms a reservoir 324 bordered on one side by weir326 so that the slurry contained within the reservoir 324 is of constantdepth. Overflow passes into section 328 from which it is withdrawn andrecirculated via level controlled valve 330 to the slurry holding tank310, the control being effected in dependence on the level of the slurrywithin section 328. Usually the slurry feed to the filter will be underpressure and at a relatively high temperature; for instance, typicalpressure and temperatures of the slurry at the inlet to the rotaryvacuum filter are about 8 bara and 170° C.

A suction unit 332 is located within the interior of the support 322 soas to exhaust fluid (both pressurising fluid and filtrate) from theinterior of the filter medium. The arrangement is such that, as thesupport 322 rotates in a clockwise direction as viewed in FIG. 5,successive parts of the periphery of the cylindrical filter dip into thereservoir 324 and slurry is drawn against the filter medium. As thefilter medium rotates, the applied suction serves to draw the liquorthrough the filter medium, forming a cake of solid material (e.g.crystals of terephthalic acid) on the upstream face of the filtermedium. The filtration process is driven by the application ofdifferential pressure across the filter medium. This pressure isdeveloped by the supply of pressurised fluid to the space between thehousing 320 and the filter medium. The pressure exerted by the fluid issuch that a small pressure differential is established through thethickness of the filter cake. Typically, this pressure differential isof the order of 0.1 to 2 bar. With reference to the typical slurrypressure quoted above, the pressurised fluid may be supplied at apressure of the order of 8.5 bara and the pressure at the downstreamside of the filter medium is typically of 0.1 to 2 bar less but at leastsubstantially the same as or greater than the pressure prevailing in thefinal stage of the crystallisation process.

At a location above the reservoir 324, the filter medium traverses awashing zone 334 in which with a washing medium such as water issuingfrom nozzles 336 is applied to the filter cake. In the (clockwise)direction of filter travel, this washing zone is preceded by afiltration zone 338 and is succeeded by a further filtration zone 340 inwhich the wash liquid and any residual liquor from the slurry is removedfrom the filter cake. The filter cake is then finally dislodged from thefilter medium in the zone 342 by unshown means and falls into acollection section 344 from which it is recovered and optionally driedto produce substantially dry pure terephthalic acid.

The fluid used to pressurise the upstream side of the filter medium isconstituted by the solvent component present in the liquor, i.e. waterwhere the slurry is derived from the purification of terephthalic acid,the solvent used for the purpose of establishing the pressuredifferential being in its vapour phase. Thus, for example, steampressurised by compressor 346 is introduced into the housing 320 vialine 348 and circulates around a path in which it passes through thefilter cake and the filter medium, enters suction unit 332 (along withfiltrate), and returns to the compressor 346 via line 347 and line 352.Filtrate removed by suction unit 332 collects in the tank 350 from whichit is withdrawn for further processing via level controlled valve 354.The supply of steam to the rotary filter is controlled by valve 356 andpressure differential sensor 358 coupled between lines 347 and 348.Excess steam may be purged, as necessary, from the system via valve 360while make-up steam may be supplied, as necessary, via valve 362 bothunder the control of pressure sensor 364. The steam supply to the systemmy be derived from any suitable source; for instance, it may beconstituted by steam derived from the purification process at a pointupstream of the filtration system, a convenient source being steamproduced during a conventional crystallisation process applied to theliquor withdrawn from the hydrogenation reactor.

The steam employed as the pressurising fluid will usually be at atemperature slightly above that at which the liquor is introduced intothe filter. Thus, for example, where the liquor is introduced at apressure of 8 bars and at 170° C., the steam admitted as thepressurising steam will be at about 173° to 175° C. and, in particular,at a temperature which ensures that there is no tendency forcondensation. The arrangement is such that the temperature of thepressurising fluid on the upstream and downstream sides of the filtercake are substantially the same and the equipment will be thermallyinsulated to avoid heat losses as the pressurising fluid circulatesaround the system. In practice, the compression of the circulating fluidby compressor 346 introduces heat into the circulating steam and meansmay be provided for moderating or controlling the temperature of thesteam entering the filter. For instance, this may be achieved by passingthe steam through a heat exchanger following compression or bycontrolled injection of further steam or water (depending on whetherheating or cooling is required) into the recirculating vapour to adjustthe temperature thereof so that the steam entering the filter is withina desired temperature range.

Referring now to FIG. 6, there is shown schematically a filtrationsystem in which a housing 400 encloses a continuous filter belt or band402 which is driven so that the upper run thereof travels from left toright as viewed in FIG. 6. The upper run of the filter band 402traverses three zones A, B and C. A slurry of terephthalic acid inaqueous liquor is introduced to the first zone via line 403 onto theband and aqueous liquor passes through the band 402 with the assistanceof pressurised fluid, namely steam (which may be derived from thecrystallisation section K or from elsewhere), the steam serving todevelop a pressure differential between the space above the upper run ofthe band 402 and the space immediately beneath the upper run thereof.The aqueous liquor passing through the band 402 in zone A is collectedin collector pans 404 located beneath the upper run of the band 402 andis removed from the pans 404 via lines 405 to leave a first wet depositwhich is then passed to the second zone B. The aqueous liquor is fed vialines 405 to a filtrate pot 406.

In the zone B, aqueous wash, for example water, is introduced via line408 and a series of sprays 410 and passes, with the assistance of thepressurised steam, through the band 402 to collector pans 412 to producea second wet deposit. The aqueous wash is removed via lines 414 and fedto the filtrate pot 406. The second wet deposit then passes to zone C inwhich it is removed from the upper run of the band 402, collected atpoint 416, recovered and then optionally dried to produce substantiallydry purified terephthalic acid. The water collecting in filtrate pot 406is removed via line 420 and level controlled valve 422 for furtherprocessing as described above in relation to stage L of FIG. 1.

Although all of the liquor collected following filtration is shown asbeing collected in a common collection vessel, i.e. filtrate pot 406, itwill be appreciated that liquor collected at different locations alongthe filter medium my be collected and supplied to different collectionpoints as required. Also, as previously mentioned, the liquor collectedat a location adjacent the end of the travel path of the upper run may,instead of being routed to the filtrate pot 406, be used to wash thedeposit at a location preceding it, and likewise the liquor collectedfrom the latter location may be used to effect washing at the locationpreceding it, and so on.

The pressurised steam is circulated around a loop including compressor424, valve 426 (controlled by differential pressure sensor 428), thesteam passing from the upstream side of the band 402 through the filtercake and the filter medium to the (downstream side) underside of theupper run of the band from where it passes to the filtrate pot 406 alongwith the filtered aqueous medium. Make-up steam is added to the systemas needed, and steam is purged from the system when necessary, by meansof valves 430, 432 controlled by pressure sensor 434. In an alternativeembodiment, purge and make-up of the steam for pressurising the systemmy be effected by means of a balance line connected to the crystalliserfrom which the slurry is obtained. In this case, the need for valves 430and 432 and sensor 434 is obviated. As described in relation to theembodiment of FIG. 5, the arrangement is such that temperature of thesteam is substantially the same on each side of the filter cake and,where necessary, the temperature of the steam entering the filter may bemoderated or controlled as previously described.

The filter system shown in FIG. 6 may be generally in the form of a beltfilter of the type which is manufactured by Pannevis BV of Utrecht,Holland and uses intermittent application of vacuum as described forinstance in the Proceedings of the Filtration Society, Filtration &Separation, March/April 1979, Page 176 et seq.

In carrying out the filtration process as described above in relation tothe embodiments of FIGS. 5 and 6, the temperature at the upstream anddownstream sides of the filter is maintained substantially the same,i.e. isothermal operation, so as to substantially prevent thedevelopment of a supersaturation condition during the filteringoperation, thereby preventing deposition of otherwise solubleconstituents of the liquor and hence reduction in product quality and/orclogging of the filter medium.

We claim:
 1. A process for the production of terephthalic acidcomprising:(a) effecting oxidation of paraxylene in a reaction mediumcomprising an aliphatic carboxylic acid to produce a first slurry ofcrude terephthalic acid in the acidic medium; (b) under elevatedpressure conditions subjecting said first slurry to an integratedfiltration and water washing process in which said acidic medium isexchanged with water to produce a water-containing calve of crudeterephthalic acid crystals; (c) dissolving the water-containing crudeterephthalic acid crystals from step (b) in water to produce an aqueousterephthalic acid-containing solution; (d) contacting the terephthalicacid solution with hydrogen under reducing conditions and elevatedtemperature and pressure conditions to reduce chemically at least someof the impurity present in the crude terephthalic acid; (e) reducingpressure and temperature to obtain a second slurry comprisingcrystallised purified terephthalic acid in aqueous medium containingreduced impurities in solution; and (f) under elevated pressureconditions subjecting the second slurry to an integrated filtration endwater washing process in which said aqueous medium is displaced from thepurified terephthalic acid by water without reslurrying the purifiedterephthalic acid prior to washing thereof.
 2. A process as claimed inclaim 1 in which the washing operation of step (b) is carried out in anumber of stages in counter-current fashion such that the crudeterephthalic acid is contacted with wash liquor comprising water ofincreasing purity as it is progressed through successive stages of thewashing operation.
 3. A process as claimed in claim 1 in which thewashing operation of step (f) is carried out in a number of stages incounter-current fashion such that the purified terephthalic acid iscontacted with wash liquor comprising water of increasing purity as itis progressed through successive stages of the washing operation.
 4. Aprocess as claimed in claim 1, 2 or 3 in which said integratedfiltration and washing processes carried out in steps (b) and (f) areeach implemented under elevated pressure conditions using a respectivepressurised belt filter.
 5. A process as claimed in claim 4 in whichsaid belt filter is operable to transport the respective slurry on theupper run thereof through a first separation zone in which the acidic oraqueous medium is separated from the terephthalic acid component of therespective slurry to leave a wet mass of crude or pure terephthallc acidcontaining residual acidic or aqueous medium on the belt filter and asecond washing zone in which wash liquor comprising water is passedthrough said mass of terephthalic acid.
 6. A process as claimed in claim5 in which each belt filter traverses a third zone in which the waterwashed terephthalic acid is removed from the belt filter and in whichthe belt filter itself is washed to remove adhering deposits during itsreturn travel from said third zone to the first zone.
 7. A process asclaimed in claim 4 in which the belt filter used for carrying out theseparation and washing process of step (f) is operated with a pressuredifferential such that the pressure prevailing on the low pressure sideof the filter is no less than said pressure prevailing following step(e).
 8. A process as claimed in claim 1, 2 or 3 in which said integratedfiltration and washing processes carried out in steps (b) and (f) areeach implemented under elevated pressure conditions using a filtersystem selected from the group consisting of a pressurised belt filter,a pressurised rotary cylindrical filter and a pressurised multi-celldrum filter.
 9. A process as claimed in claim 1, 2 or 3 in which theacidic medium and wash liquor recovered from step (b) are recycled tothe oxidation reaction of step (a).
 10. A process as claimed in claim 1,2 or 3 in which, prior to step (d), the aqueous solution of crudeterephthalic acid is subjected to oxidation to increase conversion ofterephthalic acid precursor compounds into terephthalic acid.
 11. Aprocess as claimed in claim 1 in which the heat of reaction generated instep (a) is removed by withdrawing vapour containing carboxylic acid andwater from the reaction and processing the vapour to derive a watercomponent and a carboxylic acid component.
 12. A process as claimed inclaim 11 in which the water component derived from said vapour isemployed in step (c).
 13. A process as claimed in claim 11 or 12 inwhich the water component derived from said vapour is employed as washliquor in step (f).
 14. A process as claimed in claim 1 in whichfiltrate derived from step (f) is evaporated or cooled to recover a lesspure terephthalic acid precipitate which is recycled to step (a).
 15. Aprocess as claimed in claim 1, 2 or 3 comprising maintaining adifferential pressure across the filter surface in step (f) such that,on the lower pressure side of the filter surface, the pressure issubstantially the same as or greater than the pressure prevailingfollowing step (e).
 16. A process as claimed in claim 1, 2 or 3 in whichthe pressure differential established between upstream and downstreamsides of the filter medium in step (b) or step (f) is created by meansof a pressurised fluid constituted at least in part by a solventcomponent of said medium, which component is in its vapour phase wherebyflashing of the solvent component actually present in said medium issubstantially suppressed during filtration of the crude or purifiedterephthalic acid slurry.
 17. A process as claimed in claim 16 in whichthe partial pressure of said solvent component in or forming thepressurised fluid on the downstream side of the filter medium ismaintained substantially at or above the vapour pressure of the solventcomponent actually present in the slurry fed to the upstream side of thefilter medium.
 18. A process for the purification of crude terephthalicacid, comprising:(a) dissolving the crude terephthalic acid in anaqueous medium to produce a terephthalic acid-containing solution; (b)contacting the telephthalic acid solution with hydrogen under reducingconditions and at elevated temperature and pressure to reduce chemicallyat least some of the impurity present in the crude terephthalic acid;(c) reducing pressure and temperature to obtain a slurry comprisingcrystallised pure terephthalic acid (PTA) in aqueous medium; (d) in afirst zone affecting separation of the PTA crystals from the aqueousmedium by filtration of the slurry to remove aqueous medium through afilter surface to obtain a wet mass of PTA crystals; (e) transferringsaid wet mass of crystals to a second zone without reslurrying said massand supplying aqueous wash liquor to said mass while effectingfiltration whereby the wash liquor is displaced through the mass andthrough a filter surface in the second zone; end (f) maintaining adifferential pressure across the filter surface in at least one of saidzones by means of a steam containing pressurised fluid whereby flashingof the aqueous medium is substantially suppressed during filtration ofthe slurry.
 19. A process as claimed in claim 18 in which aqueous mediumseparated in step (d) and/or wash liquor displaced through said mass isstep (e) is recycled for use in step (a).
 20. A process as claimed inclaim 18 in which said crude terephthalic acid is derived from theliquid phase oxidation of paraxylene in an aliphatic carboxylic acidsolvent in the presence of a heavy metal catalyst.
 21. A process asclaimed in claim 18 in which aqueous is medium separated in step (d) isevaporated or cooled to produce a less pure terephthalic acidprecipitate for recycle to said liquid phase oxidation of paraxylene.22. A process as claimed in claim 18 in which said crude terephthalicacid is derived from hydrolysis of a polyalkylene terephthalate.
 23. Aprocess as claimed in any one of claims 18 to 22 in which the steamforms at least the major part (by volume) of said pressurising fluid.24. A process as claimed in claim 23 in which the partial pressure ofthe steam of the pressurised fluid on the downstream side of the filtermedium is maintained substantially at or above the vapour pressure ofthe water in the terephthalic acid slurry fed to the upstream side ofthe filter medium.
 25. A process as claimed in claim 23 in which theupstream side of the filter surface with which the slurry is contactedis maintained at a pressure within the range of 2 to 15 bara and thedownstream side of the filter surface is at a pressure which is from 0.1to 10 bar lower.
 26. A process as claimed in claim 23 in which theseparation and washing process carried out in steps (d) and (e) isimplemented under elevated pressure conditions using a filter systemselected from the group consisting of a pressurised belt filter and apressurised rotary cylindrical filter.